[Views AX] Move and rename TestBrowserAccessibilityDelegate to ui layer

The TODO "Rename this class to `TestAXPlatformTreeManagerDelegate`
has existed for a while, and in preparation of moving
BrowserAccessibility and BrowserAccessibilityManager to the ui layer,
this can and should be moved ahead of time.

Moving BrowserAccessibility and BrowserAccessibilityManager to the UI
layer will allow us to share the functionality of these classes with
Views as well, allowing us to have a consistent accessibility implementation, while also enabling Views to utilize our caching
mechanisms, allowing us to stop recomputing everything on each accessibility API call to views.

Bug: 40672441
Change-Id: I75676c447f9b9de4086247f0d295e4b7d2020024
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5413917


Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Javier Contreras <javiercon@microsoft.com>
Auto-Submit: Jacques Newman <janewman@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1282716}